It's in his dream or his memories. I tried to explain it with Daphney's and Annie's conversation. The last guy is the only one who is real, the rest are just memories of the night before, which is why the other 3 are also there.
But as it's a dream that the Witches have entered, they have control. Annie's never done this before so she's playing it a little more like real life, that's why she seduced Jason, Daph knows how it works and will have fun with it.
And Carey. Oh Carey, it's her spell, she loves it and she's a gluttonous little Witch.